LARGE MOLECULAR QUADRATIC HYPERPOLARIZABILITIES IN DONOR ACCEPTOR-SUBSTITUTED TRANS-TETRAAMMINERUTHENIUM(II) COMPLEXES/

A series of new Ru(II) complex salts trans-[Ru(NH3)(4)(L-1)(L-2)](PF6) (n) [n = 2, L-1 = 4-acetylpyridine (4-acpy) and L-2 = 4-(dimethylamino )pyridine (dmap) (1), 4-(dimethylamino)benzonitrile (dmabn) (2), 4-pic oline (4-pic) (3), or 1-methylimidazole (1-MeIm) (4); n = 3, L-1 = N-m ethyl-4,4'-bipyridinium (MeQ(+)) and L-2 = dmap (6), dmabn (7), 1-MeIm (8), 4-acpy (9), or phenothiazine (PTZ) (10); n = 2, L-1 = dmap and L -2 = 4-pyridinecarboxaldehyde (pyca) (12) or ethyl isonicotinate (isne ) (13)1 have been synthesized and fully characterized. These complexes display intense, visible metal-to-ligand charge-transfer (MLCT) absor ptions which are highly solvatochromic. An X-ray crystal structure det ermination has been carried out for trans-[Ru(NH3)(4)(MeQ(+))(PTZ)](PF 6)(3) . Me2CO (10 . Me2CO). This salt, empirical formula C26H38F18N7OP 3RuS, crystallizes in the hexagonal system, space group P6(3), with (a = b = 17.853(4) Angstrom, c = 21.514(6) Angstrom, and Z = 6. The MeQ( +) ligand adopts an almost planar conformation, with a torsion angle o f 9.6 degrees between the two pyridyl rings. The dipolar cations exhib it a strong projected component along the z axis, but crystal twinning precludes second-harmonic generation. Measurements of the first hyper polarizability beta by using the hyper-Rayleigh scattering technique a t 1064 nm yield very large values in the range (232-621) x 10(-30) esu , the largest being for trans-[Ru(NH3)(4)(MeQ(+))(dmap)](PF6)(3) (7). These beta values are resonance enhanced via the MLCT excitations. A c orrelation between beta and the MLCT absorption energy confirms that t his excitation is the primary contributor to beta. The two-level model yields static hyperpolarizabilities beta(0) in the range (10-130) x 1 0(-30) esu, with trans-[Ru(NH3)(4)(MeQ(+))(dmap)](PF6)(3) (6) having t he largest. The beta(0) values of the complexes of the bipyridyl ligan d MeQ(+) are larger than those of their analogues containing monopyrid yl ligands because of extended conjugation. beta(0) correlates with th e MLCT energy only when the MLCT absorption is sufficiently far from t he second harmonic at 532 nm.